Rebecca Leighton, 27, was arrested in the early morning at her home, the Manchester Evening News reported. She is a staff nurse at Stepping Hill Hospital, about a mile from her apartment in Stockport, Greater Manchester.
Investigators said three patients died when saline solution, used for intravenous procedures, was contaminated with insulin. Two others survived.
Those who died were George Keep, 84, who was being treated for a broken hip; Arnold Lancaster, 71, a retired photographer; and Tracey Arden, 44, who had suffered from multiple sclerosis for 12 years.
The deaths had been under investigation for several days.
The Nursing and Midwifery Council said it had begun an investigation to determine if Leighton's license should be suspended.
"I can't believe it, I'm in total shock. She is just a lovely, happy person," a friend who did not want to be named told the News. "It's hard to get my our head round something like this and believe it's possible. She's just a nice, normal girl."